Recently, carbonaceous materials that do not generate lithium dendrites have been introduced for use in place of lithium metal as the negative active material for rechargeable lithium batteries. A negative electrode is produced by mixing a negative active material and a binder, and optionally a conductive material in an organic solvent to prepare a negative active material composition, and coating the composition on a current collector followed by drying.
The binder provides adhesion between the current collector and active material powders and adhesion among the active material powders when coating the active material on the current collector. In addition to good adhesion properties, desired features for the binder include good electrochemical stability, non-flammability, electrolyte-wettability, low electrode expandability, and high dispersion and crystallization degrees.
Polyvinylidene fluoride is generally used as a binder. However, polyvinylidene fluoride is a fiber which tends to cover the negative active material making it difficult for the active material to effectively perform its function. Furthermore, polyvinylidene fluoride binder has somewhat insufficient adhesion, which results in the separation of the negative active material from the current collector as charge and discharge cycles are repeated, thereby decreasing capacity and deteriorating the cycle life characteristics.
Furthermore, N-methyl-2-pyrrolidone organic solvent, which is a good solvent for polyvinylidene fluoride, tends to generate a vapor that can cause safety problems.
A binder that is suitable for an active material developed for high performance is desired. A carbonaceous material as a negative material is a chemically inactive material, but the structure and surface properties (hydrophobic or hydrophilic) of the negative material vary depending on the kind of active material and thus satisfactory adhesion is difficult to obtain. In particular, a natural graphite-based active material has a flat shape and thus its tap density and appearance density are very low resulting in deterioration of adhesion when a PVdF binder is used in a conventional amount.
Investigation into the use of styrene butadiene rubber (SBR) and polytetrafluoroethylene as binders have been undertaken. Such materials do not cause the negative active material to be covered, and they can be used in aqueous solutions such that solvent removal is not necessary. However, these materials exhibit poor adhesion compared to polyvinylidene fluoride, and do not exhibit good cycle life characteristics. In addition, SBR exhibits high expandability and tends to agglomerate in a slurry resulting in poor dispersion.